| Astronomical constraints on deposition of the Middle Triassic Chang 7 lacustrine shales in the Ordos Basin, Central China Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): Rui Zhang, Zhijun Jin, Quanyou Liu, Peng Li, Zhenkai Huang, Juye Shi, Yunjin Ge, Kefeng Du AbstractThe Middle Triassic was a key period that witnessed the evolution of Earth system processes and the commencement of a terrestrial lake in the Ordos Basin, Central China. A high-precision stratigraphic framework is the key to understanding the nature and pattern of critical geological events. Detailed time series analyses of magnetic susceptibility (MS) data were performed on the deep lacustrine shale-dominated Chang 7 Member of the Yanchang Formation from the Y1011 well core. The results reveal well documented cyclic variations with wavelengths of 5.37 m, 1.39–1.78 m, 0.48 m, and 0.24–0.30 m, which are driven by long-eccentricity, short-eccentricity, obliquity, and precession in the Middle Triassic. The stable 405-kyr tuned floating astrochronological time scale (FATS) reveals that the depositional duration of the Chang 7 Member is approximately 5 Myr, and the sedimentation rates range from 0.90 cm/kyr to 1.69 cm/kyr. In particular, the lower part of the Chang 7 Member is characterized by an organic-rich, black shale, called "Zhangjiatan Shale", whose depositional duration can be estimated at about 1.7 Myr. Along with the published biostratigraphic divisions and U |
| Are light-dark coupled laminae in lacustrine shale seasonally controlled? A case study using astronomical tuning from 42.2 to 45.4 Ma in the Dongying Depression, Bohai Bay Basin, eastern China Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): Ke Zhao, Xuebin Du, Yongchao Lu, Shipeng Xiong, Yong Wang AbstractAs a typical sedimentary structure in fine-grained rocks, laminae are widely distributed in shales and mudstones in sedimentary basins. The Shahejie Formation (42.2–45.4 Ma) formed during the Eocene of the Paleogene in the Dongying Depression of China is recognized as a typical area to study lacustrine shale. According to the composition of different lamina, four types of couplets are identified, including carbonate-clay couplets, carbonate-organic couplets, clay-organic couplets and carbonate-clay-organic triplets. All couplets are composed of light and dark layers. A combination of core images, microscopic observations, mineral compositions, geochemical data, carbon and oxygen isotopes, and strontium isotopes verifies that the laminae are primarily developed in a saline and anoxic, or even euxinic environment, with a high organic matter (OM) flux. Through astronomical cycle analysis of the natural gamma ray (GR) curve from sample site well NY1, the data suggest that the lacustrine shale laminae are formed annually with an average duration of 1.34 yr. The formation of laminae is affected by the season, which is closely related to the seasonal growth and death of algae. During the period of algae growth, a large number of light-colored carbonate laminae are deposited, whereas when the algae dies, organic matter accumulates at the bottom of lakes and forms dark organic-rich laminae. The interbedded and lenticular laminae, which are subdivided by layering characteristics, are primarily affected by diagenesis in postdepositional processes. Study of the laminae is helpful in understanding the formation process of lacustrine shale, and it provides invaluable sources of information for paleoclimate reconstruction. |
| δ13C stratigraphy of a Turinian-Chatfieldian (Upper Ordovician) foreland succession, Ottawa Embayment (central Canada): resolving local and inter-regional isotope excursions in a tectonically active basin Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): Nkechi E. Oruche, George R. Dix, Sean Gazdewich AbstractPositive δ13Ccarb excursions are correlated through an upper Turinian to lower Chatfieldian carbonate-platform succession along the axis of the Ottawa Embayment and into outliers of the northern Ottawa-Bonnechere graben in central Canada. Successive Turinian excursions (E1 and E2) are lithostratigraphically constrained by erosional surfaces and hosted within the Watertown and overlying L'Orignal formations, respectively, the latter coeval with the Selby Formation in the adjacent northern Appalachian Basin. The excursions coincide with periods of regional transgression, but geographic patterns of 13C depletion versus enrichment coincide with structurally defined areas of stratigraphically condensed and preferentially thickened formation successions, respectively. Differential subsidence is interpreted to have created bathymetric variation resulting in intrabasinal restriction of seawater exchange between these areas, with preferential Corg recycling with stratigraphic condensation. By early Chatfieldian time, segmentation of the once regional carbonate platform (L'Orignal Formation) produced a regional mosaic of low-energy muddy carbonate banks (Rockland Formation) and a deeper water platform (lower Hull Formation) settings subject to fluctuating high to low energy current flow. Excursion E3 occurs in both successions, but 13C enrichment is associated only with the bank-top muddy facies. This may identify preferential photosynthetic drawdown of 12C across the bank tops due to limited seawater exchange across the bank-deeper platform boundaries. Excursions E1 to E3, and a younger excursion (E4) in the Hull Formation, are correlated with varying confidence with excursions across southern Laurentia, excursion E3 being the local expression of the Guttenberg δ13C excursion. Our study supports local modulation of regional, if not global, δ13C excursions arising from structurally controlled changes in oceanography and productivity. |
| Addressing the effects of sampling on ecometric-based paleoenvironmental reconstructions Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): J. Tyler Faith, Andrew Du, John Rowan AbstractEcometric analysis involves the examination of quantifiable functional traits across the taxa in a biotic community. Well-documented relationships between certain functional traits and environmental gradients in the present provide the empirical framework for a large body of research that uses ecometrics to reconstruct environments in the fossil record. In current applications of the technique, the taxa present in a fossil assemblage are summarized using the mean value of an environmentally informative trait. This study explores some of the quantitative pitfalls inherent to this approach. Through analysis of dental crown height—a trait that is widely used to infer paleo-precipitation—of Late Pleistocene ungulate assemblages from the Lake Victoria Basin in western Kenya, we illustrate how ecometric means vary as a function of sample size. Sampling artifacts have the potential to bias ecometric means, and the environmental inferences derived from them, whenever there is a non-random distribution of traits across the species abundance distribution (e.g., if abundant taxa have different traits than rare taxa). This sampling issue also means that the degree of analytical precision implied by quantitative paleoenvironmental reconstructions (e.g., annual precipitation at time X was 500 mm/yr) derived from ecometrics may be unwarranted. We recommend that analytical approaches be modified to circumvent these problems and explore three potential solutions: (1) specimen-based rarefaction, (2) coverage-based rarefaction, and (3) weighting ecometric means by taxonomic abundances. Of these, only the latter is robust to variation in sampling effort. Because abundance data are not always available and are potentially unreliable, we outline alternative approaches that could be implemented to contend with sample bias. |
| Evidence of SPICE-related anoxia on the Laurentian passive margin: Paired δ13C and trace element chemostratigraphy of the upper Conasauga Group, Central Appalachian Basin Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): Justin E. Mackey, Brian W. Stewart AbstractThis study reports data from the Upper Cambrian Conasauga Group and overlying Copper Ridge Formation of the Central Appalachian region, eastern U.S.A. Geochemical, isotopic and petrographic analysis of core material was carried out to constrain the extent of marine anoxia and to characterize sediment fluxes on the carbonate platform and continental shelf of Laurentia contemporaneous with the Steptoean Positive Carbon Isotope Excursion (SPICE), a Late Cambrian global marine anoxic event. Carbonate rocks (primarily dolostone) record a positive δ13Ccarb excursion starting in the middle Nolichucky Formation, reaching its peak (+4.3‰) at the boundary between the Maynardville and Copper Ridge formations. Strontium isotope ratios in the dolostone units are only slightly offset from the expected Cambrian seawater values, suggesting minimal post-diagenetic disturbance of isotopic and trace element systematics. Selective leaching of carbonate units reveal upward decreasing trends in dissolved redox-sensitive trace metals (e.g., U, Ni, V) indicative of drawdown from regional and global marine anoxia during the peak of the Late Cambrian SPICE event. |
| Palaeoecological implications of an Upper Cretaceous tetrapod burrow (Bauru Basin; Peirópolis, Minas Gerais, Brazil) Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): Agustín G. Martinelli, Giorgio Basilici, Lucas E. Fiorelli, Carolina Klock, Joachim Karfunkel, Ariela Costa Diniz, Marcus V.T. Soares, André Marconato, João Ismael da Silva, Luiz Carlos B. Ribeiro, Thiago S. Marinho AbstractWe describe a globally rare example of a tetrapod burrow from the Upper Cretaceous Bauru Group (Bauru Basin) from Peirópolis, Minas Gerais State, Brazil. The sedimentary succession containing the burrow includes a rich vertebrate assemblage comprising fish, podocnemid turtles, mesoeucrocodylians, saurischian dinosaurs, among others. The burrow is composed of an oblique tunnel (~30°), oval in cross-section, with a horizontal and sub-oval terminal chamber; it is 1.3 m long from the midpoint of its inferred entrance to the midpoint of the bottom of the chamber. It occurs in the upper portion of a sandstone succession, interpreted as a braided channel deposit, and the burrow-fill comprises medium-grained sandstone with mudstone intraclasts derived from fluvial floodplain facies. it is overlain by other fluvial channel deposits. Analyses suggest that the burrow was dug after the filling of the braided channel and during the pedogenesis of its exposed upper surface. Based on burrow morphology and size, the most plausible producer of this burrow is a notosuchian mesoeucrocodylian, such as small to mid-sized notosuchians (e.g., sphagesaurids). The Bauru Group has an extensive fossil record of notosuchians with disparate morphologies, and it is noteworthy that the small-sized notosuchian Labidiosuchus amicum comes from the same unit as the burrow. Moreover, arid to semi-arid conditions have been inferred for fossil-bearing rocks of this unit, and as such the data here presented add to our palaeoecological knowledge of Cretaceous mesoeucrocodylians in Gondwana. Moreover, it constitutes a new Cretaceous record of a tetrapod burrow during a period when such ichnofossils are globally rare. |
| Dental histology of late Miocene hipparionins compared with extant Equus, and its implications for Equidae life history Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): Guillem Orlandi-Oliveras, Carmen Nacarino-Meneses, Meike Köhler AbstractHipparionins were a dominant element of the late Miocene faunas of Europe; however, their biology and ecology remain incompletely understood. In this paper, we explore the pace of life history of different-sized hipparionin horses, using dental histology, and compare it with extant equids. In doing so we consider (i) the size diversity of hipparionins, (ii) their generally smaller size compared to extant equids, and (iii) the allometric coupling between size and life history. In particular, we reconstruct the dental growth in lower first/second molars and in later-formed lower third molars for three hipparionin taxa: two dwarf species (Hipparion periafricanumand H. gromovae), and a larger species (H. concudense). We also analyze dental growth in an extant zebra (Equus quagga) for comparative purposes. Our results reveal that, within each species, there are differences in enamel growth parameters between the first/second molars and third molars. These results illustrate the differences in the developmental timing of these teeth and the existence of a relationship between dental growth parameters with somatic growth. We also find that hipparionin teeth grow at slower rates and tend to erupt later in time than in extant Equus. Dwarf hipparionins, moreover, exhibit lower enamel extension rates than the larger species, but similar formation and eruption times. Considering the link between dental development and life history, these results suggest a slower pace of growth of selected hipparionins compared to extant equids, and a further slower life history than expected for their size in the two dwarf forms. |
| Cyclonic activity over northeastern Africa at 8.5–6.7 cal kyr B.P., based on lacustrine records in the Faiyum Oasis, Egypt Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): Leszek Marks, Fabian Welc, Krystyna Milecka, Abdelfattah Zalat, Zhongyuan Chen, Aleksandra Majecka, Jerzy Nitychoruk, Alaa Salem, Qianli Sun, Marcin Szymanek, Izabela Gałecka, Anna Tołoczko-Pasek AbstractDuring African Humid Period in the Holocene when the summer Intertropical Convergence Zone migrated to its northernmost position, the Qarun Lake in the Faiyum Oasis in Egypt was fed with regular inflows from the Nile River and rainfall brought by the Mediterranean winter circulation. Finely-laminated lake sediments, dated at 8.5–6.7 cal kyr B.P., were examined in terms of lithology, geochemistry, microfossils (diatom, pollen) and magnetic susceptibility. Based on the inferred geographical derivation of pollen, the environmental affiliation of diatom taxa and geochemistry of lake sediments, wind trajectories were distinguished, related to two main atmospheric circulation phases. During the earlier phase (8.50–7.83 cal kyr B.P.) there were northwestern wind trajectories followed by southern ones and during the later phase (7.83–6.70 cal kyr B.P.), the northern winds were followed by northwestern and southern ones. Northwestern and northern winds brought winter rainfall and caused water turbulence in the lake, and the southern winds were associated with regional aridification. This scenario of atmospheric circulation in northeastern Africa extends significantly our understanding of key modes of climatic variability and wind trajectories in the Early to Middle Holocene (Greenlandian to Northgrippian) transition. |
| How quick was marine recovery after the end-Triassic mass extinction and what role did anoxia play? Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): J.W. Atkinson, P.B. Wignall AbstractOxygen restricted conditions were widespread in European shelf seas after the end-Triassic mass extinction event and they are reported to have hindered the recovery of marine benthos. Here we reconstruct the redox history of the Early Jurassic Blue Lias Formation of southwest Britain using pyrite framboid size analysis and compare this with the recovery of bivalves based on field and museum collections. Results suggest widespread dysoxia punctuated by periods of anoxia in the region, with the latter developing frequently in deeper water settings. Despite these harsh conditions, initial benthic recovery occurred rapidly in the British Jurassic, especially in shallowest settings, and shows no relationship with the intensity of dysoxia. A stable diversity was reached by the first recognised ammonite zone after the end-Triassic mass extinction. This contrasts with the deeper-water, more oxygen-poor sections where the diversity increase was still continuing in the earliest Sinemurian Stage, considerably longer than previously reported. Similar recovery rates are seen amongst other groups (brachiopods and ammonites). Oxygen-poor conditions have been suggested to delay recovery after the Permo-Triassic mass extinction, but this is not the case after the end-Triassic crisis. We suggest that this was because the European dysoxia was only a regional phenomenon and there were plenty of well-ventilated regions available to allow an untrammelled bounce back. |
| Ecophenotypic shape variability within Astarte (Class: Bivalvia) from the Pliocene of the Atlantic Coastal Plain, U.S.A.: A study using geometric morphometrics Publication date: 15 August 2019 Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528 Author(s): R. Philip Roberson, Michelle M. Casey AbstractThe Atlantic Coastal Plain has long been recognized as a natural laboratory useful for testing hypotheses surrounding the environmental and ecological effects on marine fauna. To conduct these studies in a rigorous manner, a reliable taxonomy must be established for the organisms within this physiographic province. In this study, we focus on the bivalve genus Astarte, which is commonly found in this region, and has many formally recognized species. We test between two competing hypotheses for the unusually high species richness of this genus: 1) taxonomy – the presence of discrete morphotypes represent valid species within this genus; and, 2) ecology – the many recognized 'species' in this genus in fact represent ecophenotypic variants of a smaller number of species, and thus richness is inflated. We test between these two hypotheses using a geometric morphometric analysis of landmarks placed on 646 shells, representing six different taxa from the Pliocene of the Atlantic Coastal Plain. We performed Procrustes transformation and Principle Components Analysis (PCA) on landmark data, allometric residuals, and outline harmonics to fully understand the variability of morphologies present. All PCA results show large amounts of overlap between all species. It is likely some of these species are valid taxonomic groups within the genus but should be classified as sub-species instead of separate species. These results provide strong support for our 'ecology' hypothesis, and suggests the genus Astarte needs revision. Future studies incorporating detailed sedimentological characteristics or ecological information (e.g., drilling frequency) will further elucidate palaeoecological and palaeoenvironmental drivers of ecophenotypic variation present in this genus. Further, the accurate identification of fossil species has important implications for our ability to reconstruct biodiversity trends in relation to the emergence of the Central American Isthmus and subsequent changes in ocean circulation. |
Anapafseos 5 . Agios Nikolaos
Crete.Greece.72100
2841026182
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.